Natural radioactivity assessment in soil samples from Kirkuk city …ijrr.com/article-1-2398-en.pdf · Volume 16, No 4 International Journal of Radiation Research, October 2018 Natural

International Journal of Radiation Research, October 2018 Volume 16, No 4

Natural radioactivity assessment in soil samples from Kirkuk city of Iraq using HPGe detector

INTRODUCTION

The soil is a major source for natural radioactivity, and it is the source for the radiation-hazard for the population and a source for migration and transfer of radionuclides into the environment. Therefore soil’s natural-radioactivity is considered as a basic indicator for radiological contamination (1). From the radiological point of view, the most important primordial radionuclides are of 238U-series (t1/2 = 4.47 × 109 years), 232Th-series (t1/2 = 1.41 × 109 years) and 40K (t1/2 = 1.28 × 109 years) (2). In general the average annual effective dose for an individual resulting from natural background radiation is estimated to be approximately 2.4 mSv (3).

The isotopes 238U and 235U are the main

radioisotopes of the natural uranium in earth crust with abundance 99.28 % and 0.72 %, respectively. 232Th is the only primordial isotope of thorium and its abundance is 100% on earth. The non-series radionuclides such as 40K can be found almost everywhere, including in human and animal tissues, soils and the oceans with various concentrations. The natural, isotopic abundance of 40K on earth is around 0.012 % (4). The emitted nuclear-radiations from naturally occurring radionuclides materials; NORMs is called terrestrial background radiation (5).

Several studies report measured and evaluated values for the level of natural radioactivity in different countries. For example, in Spain (6), in Egypt (7, 8), in Saudi Arabia (9), in Brazil (10), in Cyprus (11), in Turkey (12), in Nigeria (13, 14), in Jordan (15, 16), in Japan (17), in India (18-20),

A.H. Taqi1*, A.M. Shaker2, A.A. Battawy2

1Physics Department, College of Science, Kirkuk University, Kirkuk, Iraq

2Physics Department, College of Education for Pure Sciences, Tikrit University, Tikrit, Iraq

ABSTRACT

Background: Inspection of the level of radioactivity from radionuclides in soil is important for the assessment of the exposure to natural radiation. Materials and Methods: Ten samples from soil were collected from different sites of Kirkuk-IRAQ and the level of natural radioactivity was measured using gamma-ray spectrometry based on a high purity germanium (HPGe) detector. Results: It was found that the specific activity ranged from 27.4 to 57.0 Bq kg−1 for 226Ra, from 11.0 to 25.4 Bq kg−1 for 232Th, and from 207.4 to 516.0 Bq kg−1 for 40K. The results have been compared with the average values worldwide. The hazard indices have also been calculated. In general certain average values were found to be lower than the world average values; these included the average value of the radium equivalent activity (Raeq), the absorbed gamma dose rate (D), the external and internal hazard Hex and Hin, the gamma radiation representative level Index (Iγ) and the outdoor and indoor annual effective dose rate (AEDE) Conclusion: In general there are no harmful radiations effects posed to the population who live in the study area; however there are some spots which have values higher than the internationally allowable values. Keywords: Natural radioactivity, soil contamination, activity concentrations, HPGe detector, Radiation hazard indices.

*Corresponding authors: Dr. Ali H. Taqi, Fax: +964 7701 245785 E-mail:

[email protected]

Revised: September 2018

Accepted: October 2018

Int. J. Radiat. Res., October 2018; 16(4): 455-463

► Original article

DOI: 10.18869/acadpub.ijrr.16.4.455

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http://ijrr.com/article-1-2398-en.html

in Qatar (21), in Iran (22), and in Iraq (23-25). Few surveys of natural radioactivity have

been conducted in the Kirkuk city of Iraq which is the host of large oil fields. Therefore, the present work aims to determine the specific activity of 226Ra, 232Th and 40K in soil samples collected from selected sites of Kirkuk in order to understand the occurrence and distribution of natural radionuclides in the area under investigation and to evaluate potential health hazards. The radium equivalent activity (Raeq), the absorbed gamma dose rate (D), the external hazard (Hex), the internal hazard (Hin), the Gamma radiation representative level Index (Iγ) and the outdoor and indoor annual effective dose rate (AEDE) were also calculated and compared with the international values.

456

MATERIALS AND METHODS

A total of ten soil samples were collected from different locations in the city of Kirkuk-Iraq as illustrated in figure 1. Each sample was taken from a depth of 0~45 cm at the chosen point, and the Global Positioning System (GPS) were used for tracking the data recorded. The samples were mixed and sieved with 0.2 mm mesh, then placed in an oven at 100 oC for 24 hours for the purpose of drying the samples. The samples, then, were packed in a 1 kg Marinelli beaker, which were sealed and left for at least 4 weeks to ensure radioactive equilibrium between radon and its decay products (7). The prepared soil samples filled in a Marinelli beaker and sealed with plastic tape to prevent the escape of airborne radionuclides.

Int. J. Radiat. Res., Vol. 16 No. 4, October 2018

Taqi et al. / Natural radioactivity assessment in Kirkuk soil samples

Figure 1. Map shows the locations from where the samples were taken around the city of Kirkuk -Iraq. Google-Earth is appreciated for the facility.

The gamma-ray spectrometry analysis of the samples was carried out using an HPGe coaxial detector of crystal of 50 mm diameter which was operated under a high bias voltage, of +3000V (DC). To reduce the background radiation, a cylindrical lead shield of about 10 cm thick with a fixed bottom and a movable cover shielding the detector was used.

Energy calibration and detection efficiency

have been conducted each week to ensure that they were stable during the research period as part of quality control procedures. These two parameters of the detector were carried out by using the mixed radionuclide source (241Am, 109Cd, 57Co, 60Co, and 137Cs); energy (59.5, 88,122, 1173, 1332 and 6616 keV, respectively) of mass 441.0 g, volume 450.0 ± 4.5 cm3, and density 0.98± 0.01 g cm-3.

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The acquisition time for each sample was 7200s (dead time range between (0.06 and 0.23%). A series of γ-ray energy transition lines ranging from ~100 keV to 2614 keV, associated with the decay products of the 238U and 232Th decay were analyzed independently under the assumption of secular equilibrium of the radionuclides. 226Ra and 214Pb specific activity were used to determine the activity of 238U and the specific activity of the 232Th was determined using gamma-ray transitions lines of 228Ac and 212Pb.

The calculated specific activity of the radionuclides was obtained using equation (1) (26),

(1)

where A is the specific activity (Bq kg-1), CPS is the net peak count per second, Ɛ(abs) is the absolute gamma peak detection efficiency, Iγ is the absolute gamma intensity of the corresponding gamma-ray energy considered and m is the mass of the measured sample (kg) (27).

The Radium Equivalent Activity (Raeq)

parameter is widely used as a radiological hazard index. It is a convenient index compared with the specific activities of samples containing different concentrations of 226Ra, 232Th. and 40K. It was calculated using equation (2) (28),

(2)

Where ARa, ATh and AK are the specific activity of 226Ra, 232Th, and 40K, respectively.

The gamma-ray dose rate (D) in air at 1 m above the ground surface can be calculated using equation (3) (5, 6),

(3)

In natural environmental radioactivity situations, the effective dose is calculated from the absorbed dose by applying the factor 0.7 Sv Gy-1 (2).

The external hazard index (Hex) was determined for all analyzed samples from criterion formula (4) (29, 30),

(4)

Radon and its short-lived products are also hazardous to the respiratory system. The internal exposure to radon and its daughter progenies is quantified by the internal hazard index (Hin). It is given by equation (5) (31, 32):

(5)

The values of Hin must be less than unity for the radiation hazard to be negligible.

The gamma radiation representative level index (Iγ) is used to estimate the level of gamma radiation associated with different concentrations of some specific radionuclides, can be defined by equation (6) (33):

(6)

The Iγ can be used to estimate the level of γ-ray radiation hazard associated with the natural radionuclide in the materials. Its value must be less than unity in order to be safe from radiation hazard.

The average annual effective dose equivalent (AEDE) can be obtained from the average outdoor conversion coefficient from absorbed gamma dose rate (D). The outdoor and indoor AEDE were estimated by equations (7) and (8) (2, 5), AEDEout (mSv y-1) = D (nGy h-1) × 0.7 (Sv Gy-1) × 0.2 × 8760 (hy-1) × 10-6 (7) AEDEin (mSv y-1) = D (nGy h-1) × 0.7 (Sv Gy-1) × 0.8 × 8760 (hy-1) × 10-6 (8)

Where 8760 is hours per year. The outdoor

and indoor occupancy are 0.2 and 0.8, respectively. The corresponding worldwide value of AEDE is 0.08 mSv (5, 34).

Gamma spectrum analysis of the samples was performed with a computer-based gamma spectrometry system for qualitative and quantitative determination of gamma-emitting radionuclides. For signal processing, a preamplifier and shaping amplifier need a multichannel analyzer (MCA). The data were


457 Int. J. Radiat. Res., Vol. 16 No. 4, October 2018

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taken directly to the personal computer (PC) to be introduced using Canberra Genie 2000 software which perform data analysis as well as data acquisition.

RESULTS

Results of specific activity of 238U-series, 232Th-series and 40K radionuclides in the investigated ten samples are presented in table-1 and are shown as barcodes in figures 2-4 respectively. For 226Ra the minimum value observed was in sample S9 (27.4 Bq kg-1) and a maximum value was for the sample S1 (57.0 Bq kg-1), with an average of 40.11 Bq kg-1. For 232Th the minimum was 11.0 (Sample S6) and the maximum was 25.4 Bq kg-1 (Sample S1) with an average of 15.87 Bq kg-1. For 40K the minimum was 207.4 (Sample S6) and the maximum was 516.0 Bq kg-1 (Sample S1) with an average value of 302.82 Bq kg-1. The differences are significant in all samples.

Table 2 shows the range of specific activity and its average value in a number of countries worldwide in comparison with those obtained in

this research The radiological hazard indices obtained in

this study are shown in table 3. The value of Raeq for the samples varies from 19.559 Bq kg-1 to 114.256 Bq kg-1 with an average value of 81.182 Bq kg-1. The absorbed gamma dose rate varies from 9.462 to 54.560 nGy h-1, with an average value of 38.618 nGy h-1.

The calculated external hazard indexes (Hex) varies from 0.051 to 0.297 with an average value of the 0.210. The calculated average values are less than 1. The internal exposure by radon and its progeny was controlled by the internal hazard index (Hin). Hin ranges between 0.69 and 0.409 with an average value of the 0.286. The average values are less than 1.

The calculated Iγ values are also presented in Table 3. The values range from 0.147 to 0.850 with an average of 0.603. The calculated values for all samples were lower than the international values (Iγ < 1). The calculated indoor and outdoor AEDE values are also displayed in Table 3. The maximum of outdoor and indoor effective dose were obtained in sample S9: 0.069 and 0.267, respectively.



40K −1Bq kg

Th232 −1Bq kg

Ra226 −1Bq kg

Samples

516.0 25.4 57.0 S1

353.6 15.2 40.4 S2

250.2 14.0 52.0 S3

314.2 16.2 32.1 S4

260.2 12.7 49.6 S5

207.4 11.0 31.6 S6

251.0 14.4 32.8 S7

261.4 16.0 32.0 S8

283.0 12.8 27.4 S9 ..133 3132 2.33 S10

302.82 15.87 40.11 Average

Figure 2. Shows a comparison of the specific activity of 238U-series with the worldwide average value for the investigated soil samples.

Table 1. Shows the specific activity of 226Ra, 232Th and 40K in investigated soil

samples collected.

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Figure 3. Clarifies a comparison of the specific activity of 232Th-series with the worldwide average value for the investigated soil samples.

Worldwide Average Value

S1

S2

S3

S4

S5

S6

S7

S8

S9

S1

0

Figure 4. Shows a comparison of the specific activity of 40K with the worldwide average value for the investigated soil sample.

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)−1Bq kg( Specific Activity

40K 232Th 226Ra

Country Average Range Average Range Average Range .32 .52-32 11 3-2. 11 5-.2 (5) Egypt

.12 122-122 .5 1.2-2 22 1.2-1 (.5)USA

222 1122-2 21 ..2-1 .3 222-3 (..)China

.12 222-15 31 11-3 .. 21-. (.1)Japan

.12 2.2-112 13 112-.. .1 22-.1 (5)Malaysia

222 1.2-.1 .2 1.2-12 32 11-1 (5)India

.22 212-352 33 23-5 31 55-1 (5)Iran

312 11-112 32 12-.3 32 1.-.3 (5)Syria

.11 261-613 20 8-28 .3 16-38 (3.)Iraq

312 204-568 19 8-28 .5 15-41 (32)Iraq

2.2 322-.12 12 1-.2 11 2-3. (.1)Denmark

.52 22-122 3. .-.2 .2 12-322 (.2)Ireland

212 112-212 31 2-11 3. 5-132 (22)Poland

212 35-1.52 .. 3-312 .. .-352 (5)Spain

222 152-122 .2 .2-11 .5 .2-11 Worldwide

(5)Average

303 207-516 16 11-25 40 27-57 Kirkuk-Iraq Our Study

Table 2. Clarifies a comparison of natural radioactivity levels in the investigated soil samples of Kirkuk -IRAQ with those in other countries.

inAEDE

)1-mSv y(

outAEDE

)1-mSv y(

γI inH exH D

)1-nGy h(

eqRa )

−1Bq kg( Soil Samples

0.259 0.067 0.830 0.382 0.287 52.982 110.954 S1

0.202 0.052 0.640 0.315 0.225 41.185 86.621 S2

0.170 0.044 0.547 0.250 0.190 34.814 73.915 S3

0.046 0.011 0.147 0.069 0.051 9.462 19.559 S4

0.176 0.045 0.560 0.282 0.198 36.071 76.385 S5

0.201 0.051 0.640 0.310 0.225 40.999 87.100 S6

0.195 0.050 0.630 0.273 0.215 39.926 83.462 S7

0.206 0.053 0.663 0.293 0.227 42.177 87.271 S8

0.267 0.069 0.850 0.409 0.297 54.560 114.256 S9

0.166 0.043 0.525 0.275 0.188 34.001 72.303 S10

0.189 0.048 0.603 0.286 0.210 38.618 81.182 Average

0.42 0.08 ≤1 ≤1 ≤1 55 370 Recommended

value

Table 3. Radiological hazard indices obtained in this study for the city of Kirkuk-Iraq. Radium equivalent activity (Raeq), the absorbed gamma dose rate (D), the external (Hex) and internal (Hin) hazard index, the radioactivity level

index (Iγ) and the annual effective dose annual equivalent (AEDE) for the investigated soil.

DISCUSSION

The results displayed in table 2 shows that the obtained average values fall within the range of corresponding values published for other locations in the world (5, 35-40). The concentrations have been reported on a dry soil

samples. The activity concentration of 40K in soil is an order of magnitude higher than that of 226Ra and 232Th as it is the most abundant radioactive element under consideration. On the basis of the higher levels reported for China and the USA, the Committee revised the values for both 232Th to 40 Bq kg-1 in the UNSCEAR 1993

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Report (2). The results of west Asia (Iran (5), Syria

(5) and Iraq (23)) were found lower than the content detected in North America (USA) (35), and East Asia (China (36), Japan (37) and India (5)).

The world average specific activity of 226Ra is 35 Bq kg-1 within the range of 17- 60 Bq kg-1, for 232Th is 30 Bq kg-1 within the range of 11-64 Bq kg-1 and for 40K is 400 Bq kg-1 within the range of 140-850 Bq kg-1. In some samples the specific activity for 226Ra and 232Th for the investigated sites were higher than those reported in (5, 41) for radioactivity levels of 226Ra and 232Th internationally. The differences in values are significant. The high recorded values of the radionuclides in some soil samples attributed to the geochemical composition and origin of soil types in a particular area and may be due to the presence of radioactive rich granite, phosphate, sandstone, and quartzite.

Correlation between the radionuclide concentrations has been studied. It was found that 232Th has a weak correlation with 226Ra (correlation coefficient 0.319) (figure 5). However it is seen that 40K and 232Th are positively correlated (correlation coefficient 0.78) (figure 6).

It was inferred that for all the soil samples analyzed, the Raeq value was well within and less the permissible limits of 370 Bq kg-1 (6). All the obtained D values were lower than the internationally recommended value 55 nGy h-1 (5). The calculated values of Hex, Hin and Iγ for all the investigated samples were lower than the international values. It can be seen that the indoor and outdoor AEDE values were lower than the corresponding worldwide values of 0.08 and 0.42 mSv, respectively (5).



Figure 5. Correlations of activity concentration between 226Ra and 232Th.

Figure 6. Correlations of activity concentration between 40K and 232Th.

CONCLUSION

Most of the specific activity values were lower than the world average values with some exceptions which are given in figures and tables of this study, so we should focus on these sites by collecting more samples and greater depths to give a more accurate description of the area. The high recorded values in some samples may be due to the geochemical composition and origin of soil types. The averages values of the obtained radiological effects such as: the radium

equivalent activity (Raeq), the absorbed gamma dose rate (D), the external (Hex) and the internal (Hin) hazard indices, the radioactivity level index (Iγ) and the annual effective dose equivalent (AEDE) were within the limit of the internationally recommended values. It was concluded that no harmful radiation effects were posed to the population who live in the study area.

Conflicts of interest: Declared none.

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Natural radioactivity assessment in soil samples from Kirkuk city …ijrr.com/article-1-2398-en.pdf · Volume 16, No 4 International Journal of Radiation Research, October 2018 Natural